Making a successful forum

How do you come up with a topic for a forum and make it prosper? I want to make a forum using this software but I am having trouble finding a unique topic that will also help my forum stay alive and not be buried thanks to all those bigger and more well-known forums.

GFX (I LOVE gfx), Some games like Minecraft. I ran a GFX site once and it had a good run but I had to close it down thanks to it slowly dying... Now let's say I do make another GFX how would I make it stand out?